1. Field of the Invention
The invention relates to a lithium-ion secondary battery and manufacturing method thereof.
2. Description of Related Art
A nonaqueous electrolyte secondary battery such as a lithium-ion secondary battery is used as a so-called portable power supply for a personal computer, a portable device or the like, or as a power supply for driving a motor vehicle. In particular, a lithium-ion secondary battery, which is small in size and light in weight and which is capable of obtaining a high energy density, is desirably used as a power supply for driving a hybrid vehicle, an electric vehicle, or the like.
In one typical form of the lithium-ion secondary battery, a flat wound electrode body and a nonaqueous electrolyte are provided inside a rectangular box-shaped case corresponding in shape to the wound electrode body. By employing this form, it is possible to increase the capacity of individual batteries and to efficiently dispose a plurality of batteries in a limited space. Thus, this form is desirably employed in an application where an especially high energy density is required (e.g., a power supply for driving a motor vehicle).
This high-energy-density battery is usually used in such a state that a voltage is controlled to fall within a predetermined range (e.g., 3.0 V to 4.2 V). However, if currents which are above normal are supplied due to an erroneous operation and so forth, the battery may be, overcharged to a voltage exceeding a predetermined voltage. When such an overcharge occurs, a gas may be generated due to decomposition of a nonaqueous electrolyte or the internal temperature of the battery may be increased due to heat generated from an active material. As a technology for addressing this problem, PCT International Publication No. WO 2013/108396 discloses a lithium-ion secondary battery which includes a nonaqueous electrolyte containing a compound (typically, an aromatic compound) (hereinafter referred to as a “gas generating agent”) lower in oxidative decomposition potential than a nonaqueous solvent and includes a current interrupt device (CID) for forcedly interrupting a charging current if an internal pressure of a case becomes equal to or greater than a predetermined value due to decomposition of the gas generating agent. If the battery becomes overcharged, the gas generating agent is oxidized and decomposed on a surface of a positive electrode, at which time a hydrogen gas (H2) is generated from a negative electrode. The internal pressure of the battery is rapidly increased by the gas thus generated. This makes it possible to operate the current interrupt device at an early stage. Thus, it is possible to realize a battery which is high in reliability (overcharge resistance).
In the lithium-ion secondary batteries disclosed in PCT International Publication No. WO 2013/108396, if the potential of the positive electrode becomes extremely high, for example, if the potential of the positive electrode locally exceeds the elution potential of a metal element (typically, a transition metal element) which makes up a positive electrode active material, the metal element may be eluted from the positive electrode active material and may be precipitated on the surfaces of the separator and the negative electrode which are opposite to the positive electrode active material. According to the findings of the inventors, micro short-circuits may be resultantly generated inside the battery and a self-discharge amount (a leak current) may grow larger.